A typical hydrostatically driven vehicle includes an engine having an output shaft connected to one or more fluid pumps. A variable displacement fluid pump is connected to the engine through a transmission and varies its displacement to accommodate power demands of various vehicle systems. The fluid pump or pumps associated with the vehicle is connected via fluid pressure lines to various actuators and hydraulic motors around the vehicle. For example, a vehicle may have one or more hydraulic propel motors that rotate the drive wheels or rollers and move the vehicle along a base surface. Further, the vehicle may have one or more implements performing various functions. In the case of a soil compactor, an implement may be a vibrator device included within a compacting drum of the vehicle. The vibrator may operate to impart compacting energy to the base surface, thus increasing the efficiency of compaction. The vibrator and other types of rotary hydraulic actuators are powered by a pressurized flow of fluid from the pump. The speed, torque output, and power consumption of these motors are proportional to the flow rate and pressure of the hydraulic fluid passing through them.
An engine on a hydrostatically driven vehicle often typically operates in a continuous fashion at a constant speed and fueling command or, stated differently, at a constant power output. Even though the power output of the engine is constant, the power used or consumed by the various hydraulic systems of the vehicle may change based on operator commands or environmental conditions. For example, a vehicle will consume more power when travelling at a faster speed, or may impart more energy to the surface being compacted when the surface is wet.
To control the power supplied to the various systems and actuators of the vehicle, the flow rate of hydraulic fluid supplied to the various systems of the vehicle is modulated. This modulation may be accomplished, for example, through control of the displacement of the pump or selective routing of fluid to the various motors and actuators of the vehicle. However, most vehicles operating under most conditions do not entirely consume the power generated by their engines. As is often the case, an appreciable percentage of the power generated by the engine is unused and wasted. This mode of operation causes reduced fuel economy and increased noise during operation.